Patent GB-274561-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/00
Filed: 1926-04-21
Abstract: 274,561. Standard Telephones & Cables, Ltd., Lowry, L. P., Gould, J. R., and Cameron, A. H. April 21, 1926. Automatic exchange systems.-In system in which two hunting switches are permanently associated and hunt in turn for a free line in a large group of lines, the switches are advanced by the interaction of their stepping magnets with a common stepping relay, The device is especially adapted for giving access to a large group of private branch exchange lines which are called by the same number of digits as ordinary lines, the first superfluous digit being absorbed by the device and its conclusion initiating the hunting operation, and the remaining digits being rendered ineffective. On initiation of the hunting operation both switches are advanced one step, the first switch then proceeds to hunt alone and if it fails to find an idle line it stops when it arrives back on its normal terminals to which are connected the wipers of the second switch which then takes up the hunt. Means are provided for giving a busy signal and operating an overflow meter if the second switch arrives on its final terminals without finding an idle line. Selection of an'idle line. The line relav A energizes release relay B, during the first impulse series relays SD, HC being energized, the latter relay looking over its lower armature and shortcircuiting the lower winding of a relay HS. At the end of this digit relay SD relapses and removes the shunt from relay HS which responds over its lower winding in series with relay HC, locks up independently over both its windings, transfers the circuit of relay HC over the interrupter armatures of the stepping magnets DMA, DMB, and energizes these magnets over their homing wipers N and normal contacts. The relay HS remains locked up until release and renders further digits ineffective. The energization of the magnets DMA, DMB opens the circuit of stepping relay HC which de-energizes the magnets causing both switches to advance one step. The relay HC is now re-energized, but completes another circuit for the magnet DMA only over its wiper N. This magnet and the stepping relay HC continue to interact until an idle line is found. The relay HC is made slow-to-act and quick-to-relapse to ensure the correct response of the stepping magnets, When the test wiper P engages an idle line, a test relay KA opens the circuit of relay HC to prevent further operation and energizes a switching relay K, which is made slow-to-act to allow time for the cut-off relay of the seized line to pull up. If all the lines of the first switch are busy, when it arrives at its normal position the second switch is connected up- and a circuit is closed for its magnet DMB which interacts with relay HC in the same manner. Busy signal and statistical metering. When the wipers of the second switch arrive on their final terminals, relay SD energizes over its upper winding in series with relay KA, applies busy tone and flash BF to the lower conductor over the lower winding of relay A, operates an overflow meter over wire OM, and holds the release relay B over its upper winding. Ringing current. The seized line is rung over relay F which on reply locks up and completes the connection back to a supervisory relay D. Release. On the calling party hanging up, relays A, B, HS, K relapse. Between the relapse of relays Band HS ground is momentarily removed from the release wire to release the preceding switches, and on the relapse of relay US a homing circuit is closed for the off-normal switch or switches during homing the slowrelease relay SD being energized over the interrupter armatures and its upper and lower windings respectively during the restoration of the first and second switches, and holding the release wire earthed. Specifications 247,742 and 255,158 are referred to.